Environmental control systems such as heating, ventilating and air conditioning (HVAC) systems are well known and are designed and implemented to maintain environmental conditions within buildings. A typical installation sees the building divided into zones, and the HVAC system is adapted to maintain each of the zones within predefined environmental parameters (e.g., temperature, humidity, outdoor-recirculated air ratio, etc.). In this exemplary installation, an air distribution system connects each of the zones with an air handling unit (AHU) for providing a supply of conditioned air to the particular zone. The AHU generally includes elements for introducing outdoor air into the system and for exhausting air from the system; elements for heating, cooling, filtering and otherwise conditioning the air in the zone, as well as elements for circulating the air within the zone's air distribution ducts at a desired flow rate.
Air flow from the air handling unit to each room within the zone is regulated by a separate variable air volume (VAV) terminal unit, also called a VAV box. The typical variable air volume terminal unit has a damper driven by an actuator to vary the flow of air from the zone's air distribution duct. The variable air volume terminal unit also may have a heating element to increase the temperature of the air that flows in to the associated room. The operation of the variable air volume terminal unit is provided by a VAV controller that must provide both continuous and discrete control functions. Continuous control includes a temperature loop and a flow loop, while discrete control includes the sequencing of the heating and cooling devices.
Most VAV controllers cannot perform adequate hybrid control. A pneumatic VAV controller can control a simple continuous loop, but cannot perform the discrete control. Analog electric and Direct Digital Control (DDC) controllers hold great promise, but often inherit the inadequate design concepts from their pneumatic predecessors. Existing VAV controllers often cycle between heating and cooling, which results in occupant discomfort, excessive energy usage, and premature equipment failure. The use of spring-ranging to sequence heating and cooling devices, a carryover from pneumatic controls, makes it difficult to identify the mode of operation, i.e., whether the controller was cooling, heating or other. Diagnostics for cooling and heating was difficult to implement correctly.
For example, FIG. 1 shows a flow chart for a prior art sequencing control strategy 10 which may be implemented in an HVAC system controller. Control strategy 10 is based on strategies used in pneumatic control systems. A single feedback controller 12, usually a proportional-integral (PI) controller, is used with this strategy, in conjunction with economizer logic 14 and low select logic 16, to reduce component costs. The controller output is determined by comparing the supply air temperature to a setpoint. If the scaled output from feedback controller 12 is between 100% and 200%, mechanical cooling via cooling coil 32 is used to cool the air. Here 100% represents no mechanical cooling and 200% represents maximum mechanical cooling. If the outdoor air conditions are suitable, an economizer cycle 14 (outdoor air dampers fully open) is used simultaneously to reduce the mechanical cooling load. If the output from feedback controller 12 is between -100% and 0%, heating coil 30 is used to heat the supply air and the outdoor air damper is at its minimum position determined by ventilation criteria. If the output from the feedback controller is between 0% and 100%, outdoor air and return air are mixed in mixed air plenum 14 to produce supply air at the setpoint temperature. This is referred to as free cooling because neither mechanical heating or cooling is used.
The dynamic characteristics of the three processes (i.e., heating, cooling, and free cooling) are significantly different, in which case the use of a single feedback controller is limiting. To maintain stable control, the controller must to be tuned for the worst case conditions. If this is the case, the closed loop response for other conditions will tend to be sluggish. If the feedback controller is not tuned for the worst case conditions, then valves and dampers of the system may cycle between fully open and fully closed with resultant energy waste and component wear.
The control performance can be improved by using an adaptive controller such as disclosed and described in commonly assigned U.S. Pat. Nos. 5,355,305, 5,506,768 and 5,568,377 the disclosures of which are hereby expressly incorporated herein by reference, to adjust the proportional gain and integral time of controller 12. However, the parameters may need significant adjustment as the control component changes, that is, as the control changes from cooling to heating. Also, it may be difficult to tune at the transition region because the combined process may be very nonlinear. During the time period that the adaptive controller is adjusting parameters at the transition region, the control performance may be sacrificed.